System and method for controlling voltage of fuel cell

ABSTRACT

This specification describes a system and method for controlling the voltage produced by a fuel cell. The system involves providing a bypass line between an air exhaust from the fuel cell and an air inlet of the fuel cell. At least one controllable device is configured to allow the flow rate through the bypass line to be altered. A controller is provided to control the controllable device. The method involves varying the rate of recirculation of air exhaust to air inlet so as to provide a desired change in fuel cell voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/892,888, filed Nov. 20, 2015, which is a National Stage Entry ofInternational Application No. PCT/CA2014/050175, filed Mar. 6, 2014,which is a non-provisional application of U.S. Application Ser. No.61/827,318, filed May 24, 2013, all of which are incorporated byreference.

FIELD

This specification relates to fuel cell power systems.

BACKGROUND

A fuel cell produces voltage according to a polarisation curve. Thepolarisation curve describes the fuel cell voltage as a function of thefuel cell current or the fuel cell current density. In general, ascurrent supplied by the fuel cell increases from zero, the fuel cellvoltage initially drops rapidly through an activation region, then dropsnearly linearly through an ohmic region, then drops more rapidly througha mass transport region. In many cases, it would be desirable to alterthe polarisation curve of the fuel cell.

INTRODUCTION

This specification describes a system and method for controlling thevoltage produced by a fuel cell. The system involves providing arecirculation line between an air exhaust from the fuel cell and an airinlet of the fuel cell. At least one controllable device is configuredto allow the flow rate through the recirculation line to be altered. Acontroller is provided to control the controllable device. The methodinvolves varying the rate of recirculation of air exhaust to air inletso as to provide a desired change in fuel cell voltage.

FIGURES

FIG. 1 is a schematic drawing of a fuel cell power system.

FIG. 2 is a schematic drawing of the fuel cell power system of FIG. 1wired in parallel with a battery and connected to a load.

DETAILED DESCRIPTION

FIG. 1 shows a fuel cell power system 10, alternatively call a fuel cellpower module. The system 10 includes a fuel cell stack 12, an air blower14, an air inlet line 16, a blower inlet line 18, an air outlet line 20,a recirculation line 22 (alternatively called a bypass line), an exhaustvalve 24, a recirculation valve 26 (alternatively called a bypassvalve), a voltage sensor 28, a controller 30 and a temperature sensor32. The system 10 also contains several other conventional elements,such as a hydrogen supply, that are not shown in FIG. 1 to allow themore material elements of the system 10 to be emphasized. Theconfiguration of the elements in system 10 may be altered. For example,there might be only one of the exhaust valve 24 and recirculation valve26. In another example, an equivalent system might be arranged with theair blower 14 attached to the air outlet line 20.

In operation, hydrogen and air flow through the fuel cell stack 12. Someor all of the hydrogen is consumed by reacting with oxygen from the airin the fuel cell stack. An excess of air, relative to the amount of airthat would carry a stoichiometric amount of oxygen to react with thehydrogen, flows through the fuel cell stack 12. The excess air serves toremove moisture from the fuel cell stack and to help ensure that localareas within the fuel cell stack are not starved of oxygen. The amountof excess air, measured at the inlet, is typically in the range of 1.5to 3 times the amount of air that would carry a stoichiometric amount ofoxygen. For brevity, this excess amount of air will be described as “Ntimes the stoichiometric amount”.

In some cases, the voltage produced by the fuel cell is not particularlyimportant. However, in other cases, the variation of voltage withcurrent according to the polarisation curve causes problems. Forexample, as shown in FIG. 2 , a fuel cell stack 12 is often connected inparallel with a battery 40 to provide a hybrid electrical power supplysystem 44. As current drawn from the hybrid system 44 by a load 42changes, the difference in the polarisation curves of the battery andthe fuel cell can cause either the battery 40 or the fuel cell stack 12to over-contribute to the total power demand. Either the battery 40 orthe fuel cell stack 12 may overheat. Therefore, it would be desirable tobe able to cause the fuel cell stack 12 to operate as if it had apolarisation curve more nearly like that of the battery 40. In othercases, a device powered by a fuel cell has characteristics that wouldmake a particular polarisation curve desirable. In some cases, it wouldbe desirable to have a more nearly flat polarisation curve. In othercases, it would be desirable for the slope of the polarisation curve inthe ohmic region to more nearly constant or to extend further into theactivation region. In these cases, creating the desired voltage outputis sometimes achieved by using a DC to DC voltage converter.

In the system 10, air can be permitted to flow through the recirculationline 22 by opening recirculation valve 26. With recirculation valve 26at least partially open, the difference in pressure between the airoutlet line 20 and the suction side of air blower 14 causes flow in therecirculation line 22. The flow through recirculation line 22 can beincreased by opening recirculation valve 26 or decreased by closingrecirculation valve 26. When the recirculation valve 26 is at leastpartially open, closing exhaust valve 24 increases flow in therecirculation line 22 and opening exhaust valve 24 decreases flow in therecirculation line 22. Accordingly, one or both of exhaust valve 24 andrecirculation valve 26 can be modulated to vary the flow rate in therecirculation line 22. Although modulating valves 24, 26 may affect thetotal head loss incurred by blowing air through the fuel cell stack 12,and therefore the total flow rate of air through the fuel cell stack 12,the change in total flow rate is small and not of primary importance tothe process. The more important result of modulating one or more ofvalves 24, 26 is that the partial pressure of oxygen in the air side ofthe fuel cell stack 12 can be altered. This effect will be illustratedin the example below.

Ambient air has about 20.9% oxygen. To simplify the followingdiscussion, it will be assumed that ambient air is 20% oxygen and 80%nitrogen and all percentages in the following sentences will beproportion to these amounts in ambient air. If ambient air is providedto the fuel cell stack 12 at 3 times the stoichiometric amount, then theincoming air has a total flow rate of 300% consisting of 60% oxygen and240% nitrogen. One stoichiometric amount of oxygen, or 20%, is consumedby hydrogen as the air passes through the fuel cell stack 12. Theexhaust gas produced has a flow rate of 280% made up of 40% oxygen and240% nitrogen. The ratio of oxygen to total gas in the exhaust gas isnow 1:7 rather than 1:5 for the ambient air. If some or all of theexhaust gas passes through the fuel cell stack again, even lower ratiosof oxygen to total gas are achieved although partially attenuated by theneed to provide make-up air. Systems that operate at lower total airflow rates relative to the stoichiometric amount can achieve evengreater variation in oxygen to total gas ratio.

Modulating one or more of valves 24 and 26 controls the relative amountsof exhaust gas and ambient air that flow through the fuel cell stack 12.This in turn controls the ratio of oxygen to total gas in the air sideof the fuel cell stack. This ratio in turn causes a change in partialpressure of oxygen in the air side of the fuel cell stack. The voltageof the fuel stack 12 varies more with the partial pressure of oxygenthan with the total gas flow. A higher oxygen partial pressure producesa higher voltage at a given current output while a lower oxygen partialpressure produces a lower voltage at a given current output. Generallyspeaking, changing the partial pressure of oxygen changes the shape ofthe polarisation curve. Despite the change in partial pressure, totalgas flow rate through the fuel cell stack remains at or above theminimum rate chosen for water removal. Further, provided that the amountof oxygen in the exhaust/ambient air blend does not result in a flow ofoxygen less than one stoichiometric amount, the high total gas flow ratehelps deliver enough oxygen to all parts of the fuel cell stack 12.

To enable real time control, one or both of the valves 24 and 26 areconnected to a controller 30. The controller 30 may be programmed tomodulate one or both of the valves 24 and 26 in a pre-programmed mannerbased on a stored formula or table giving the valve movements predictedto give a voltage desired under different operating conditions.Optionally, the controller 30 is connected to a voltage sensor 28connected to the fuel cell stack 12 to allow for a feedback loop toallow one or both of the valves 24 and 26 to be trimmed in response to avariance between desired and actual voltage. Further optionally, thecontroller 32 may be connected to a temperature sensor 32 to provide anemergency avoidance feature. If the fuel cell stack 12 is in danger ofoverheating, the controller 30 can module one or both of the valves 24and 26 to reduce the cell stack voltage.

Alternatively, the controller 30 may be part of, or connected to, alarger system. In this case, a desire for a change in the fuel cellvoltage can be determined by any part of the larger system. For example,if the fuel cell stack 12 is connected in parallel with a battery, thebattery voltage can be communicated to the controller 30 which thenmodulates one or more of the valves 24 and 26 to more nearly match thevoltage of the battery, to move towards the voltage of the battery, orto be within a range relative to the voltage of the battery. In thiscase, the voltage sensor 28 can still be used to provide an innercontrol loop operating inside of the outer control loop of the largersystem.

References to a battery above include a bank or other assembly ofbatteries connected wired in parallel or in some combination of seriesand parallel with the fuel cell stack 12. Similarly, a system 10 mayhave multiple fuel cell stacks 12, or multiple systems 10 may beassembled into a bank or other structure and then connected to one ormore batteries. In this case, one or more than one of the fuel cellstacks 12 or systems 10 may be controlled to control the voltage of theassembly as a whole to more nearly match the voltage of one or morebatteries connected in parallel. The method and apparatus describedherein can also be used when connecting a fuel cell power system 10 to aload without a battery, for example directly or through a DC-DC voltageconverter. The method and apparatus can help keep the voltage of thefuel cell power system 10 within a range required by the load, forexample computer servers or other electronic equipment, for example in aDC grid data center.

1. A method comprising, a) providing a fuel cell stack having an airinlet and an air exhaust and a recirculation line between the airexhaust from the fuel cell and the an air inlet of the fuel cell; and,b) recirculating a portion of the air exhausted through the air exhaustto the air inlet though the recirculation line.
 2. The method of claim 1further comprising varying the rate of air flow in the recirculationline of so as to provide a desired change in fuel cell stack voltage. 3.The method of claim 2 wherein the desired change in the fuel cell stackvoltage is to more nearly match the voltage of one or more batterieswired in parallel with the fuel cell stack or a portion thereof.
 4. Themethod of claim 1 wherein the rate of air flow in the recirculation lineis altered by controlling a valve.
 5. The method of claim 1 comprising astep of polling a voltage sensor connected to the fuel cell stack.